There has been an increasing demand for downsizing, weight lighting and operable frequency raising in portable mobile communication devices due to advance of information-oriented society. A compact and light acoustic wave filter is used to meet the increasing demand. Particularly, an increasing number of portable phones adopt a system in which the transmission frequency and the reception frequency are close to each other. It is thus required to realize greater attenuation at frequencies close to the pass band.
An exemplary acoustic wave filter is a surface acoustic wave filter in which an IDT composed of comb electrodes and reflectors are provided on a piezoelectric substrate. Power is applied to a SAW element and an acoustic wave is thus excited. The SAW filter is capable of processing a radio signal in the range of 45 MHz to 2.0 GHz. The SAW filter is used to form a transmission bandpass filter or a reception bandpass filter.
FIG. 1A is a plan view of a conventional double-mode SAW filter, and FIG. 1B is a plan view of a double-mode SAW filter described in Japanese Laid-Open Patent Publication No. 2000-196399. As illustrated in FIG. 1A, three IDTs 2, 4 and 6 are formed on a piezoelectric substrate 22, which may be made of, for example, lithium niobate (LiNbO3) or lithium tantalate (LiTaO3). Reflectors 8 and 10 are interposed at both sides of an alignment of the IDTs 2, 4 and 6 in the direction of SAW propagation. The IDTs 2, 4 and 6 and the reflectors 8 and 10 are made of a metal such as aluminum (Al). An intentionally reduced number of electrodes fingers is illustrated in FIGS. 1A and 1B for the sake of simplicity.
When an electric signal is applied to the double-mode SAW filter, the acoustic waves are excited by the IDTs and are propagated in a direction perpendicular to the direction in which the electrode fingers extend. The acoustic waves are converted into electric signals of frequencies of the acoustic waves. The reflectors 8 and 10 function to confine the acoustic waves propagated from the IDTs 2, 4 and 6 by utilizing reflection, so that the acoustic waves can be suppressed from being attenuated. Actually, the reflectors 8 and 10 reflect acoustic waves (spurious waves) outside of the pass band. The spurious waves are superimposed and degrade the out-of-band attenuation of the double-mode SAW filter.
As illustrated in FIG. 1B, the above-mentioned publication shows a double-mode SAW filter in which the electrode fingers of the reflectors 8 and 10 become shorter in a reflection attenuating region B as the electrode fingers are farther away from the IDTs. The reflection attenuating region B has reflectivity different from that of a reflection region A in which the electrode fingers of the reflectors 8 and 10 have an equal length. This difference causes random reflection, which cancels the spurious waves and increases the amount of attenuation at a low-frequency side of the pass band and close thereto.